which can be investigated in multi-differential manner, like for different particle species, and as a function of the centrality, the rapidity and the transverse momentum.
Here \(\phi \)  measures the azimuth (w.r.t.  to the beam axis) relative to the event plane.
The second component \(v_2\)  (“squeeze”) is particularly interesting.
As a function of beam energy the sign of \(v_2\)  changes two times.
At low beam energies, in the fusion-fission or multi-fragmentation regime, fragment and nucleon emission is preferred in plane and caused by attraction and resulting angular momentum of the combined nuclear system.
For BEVALAC, SIS18, AGS energies, \(v_2\)  turns negative (out of plane) because the projectile and target like residues do not separate fast enough from the fireball and block the in-plane emission of particles from the hot and dense zone.
At ultra-relativistic energies, \(v_2\)  is positive again and a direct measure for the pressure of the initial, almond-shaped QGP droplet [1]}.
The change of \(v_2\)  as a function of the collision energy reveals collective behavior of the matter formed in heavy-ion collisions at any energy between \(\sqrt{s_{\mathrm {NN}}}\, = 2 \, \mathrm {G}e\mathrm {V}\)  and \(\sqrt{s_{\mathrm {NN}}}\, = 5 \, \mathrm {T}e\mathrm {V}\) .
Moreover, in [2]} it has been conjectured that \(v_1\)  would signal a “softest point” if QCD feature a first order phase transition to a deconfined medium in the region of high net-baryon density.
At the same time \(v_4\)  would exhibit a maximum [3]}.
Yet, the quantification of the pressure is model dependent [4]} and the quest for the precise knowledge of the initial condition a key program of the heavy-ion community.
We will come back to the consequences of collective effects for the emission of dileptons in Sec. REF  and .
